(i) Field of the Invention
The present invention relates to a 2-cyanoacrylate-based adhesive composition which is excellent in storage stability and hardening properties of thick films and which has a high hardening rate and remarkably improved surface hardening properties of the adhesive composition itself bulged from between adherends, and it also relates to a process for preparing the 2-cyanoacrylate-based adhesive composition.
(ii) Description of the Prior Art
A 2-cyanoacrylate has been used as an adhesive which can instantaneously harden with the aid of a trace amount of moisture on the surfaces of adherends, and it is suitable for the adhesion of rubbers, plastics, lumber and the like.
The 2-cyanoacrylate-based adhesive composition closely sandwiched between the adherends can harden rapidly, but a part of the adhesive composition bulged from the adherends or exposed to air takes a long time to harden. In addition, the 2-cyanoacrylate vaporizes until the completion of the hardening, so that a user suffers a strong irritant odor, and the vaporized 2-cyanoacrylate hardens in the vicinity of the adherends to inconveniently whiten their peripheries.
As methods for most simply solving these problems, there are a method which comprises first applying a primer or a hardening accelerator comprising a basic substance onto the adherends, and then applying the 2-cyanoacrylate thereonto, and a method which comprises applying the hardening accelerator onto the bulged portion after the adhesion to rapidly complete the hardening of the bulged portion.
On the other hand, in order to inhibit the strong irritant odor and the whitening, a method has been employed which comprises introducing an alkoxyalkyl group or a long-chain alkyl group into an ester moiety of the 2-cyanoacrylate to restrain the vaporization of the 2-cyanoacrylate.
Furthermore, Japanese Patent Application Laid-open No. 1285/1992 suggests a method which comprises adding diethyl carbonate to the 2-cyanoacrylate to relieve the odor.
In addition, as the technique for improving a hardening rate, Japanese Patent Application Laid-open Nos. 152033/1979, 129231/1978 and 179482/1985 suggest methods which comprise adding polyethylene glycol, polypropylene glycol, crown ether or a calixarene compound.
However, any of the above-mentioned methods give poor storage stability, and they are not sufficiently satisfactory in points the hardening of the bulged portion, surface hardening properties and thick-film hardening properties.
That is to say, the method using the primer or the hardening accelerator can achieve (1) the surface hardening properties, (2) the thick-film hardening properties, (3) the decrease of the irritant odor and (4) the reduction of the whitening by the simplest procedure, but the one-pack type 2-cyanoacrylate-based adhesive composition becomes a two-pack type owing to the use of the primer or the hardening accelerator, so that the adhesive composition is difficult to handle and thus it is not practical.
In the method which comprises introducing the alkoxyalkyl group or the long-chain alkyl group into the ester moiety of the 2-cyanoacrylate in order to relieve the strong irritant odor and the whitening, the distillation temperature is raised at the time of the manufacture of the 2-cyanoacrylate. In consequence, the manufacture of the 2-cyanoacrylate is difficult and its storage stability is bad, and the cost increases inevitably because of the deterioration of yield.
Moreover, the method which comprises adding polyethylene glycol, crown ether or the calixarene compound can exert a sufficient effect for the improvement of the adhesion rate of the 2-cyanoacrylate closely sandwiched between the adherends, but this method inconveniently impairs the storage stability and cannot improve the hardening properties of the bulged portion, the surface hardening properties and the thick-film hardening properties.
Thus, it has been desired for a long time to develop the 2-cyanoacrylate-based adhesive which can remarkably accelerate the hardening of the bulged portion and which can minimize the irritant odor generated from the bulged portion and the whitening without introducing any specific ester group.